These monsters are an analogy to asymmetric twins. One twin (autosite) is well developed and is connected to the other one (parasite), which consists of seriously malformed tissues and organs.
Epignathus parasite is in the oral cavity of the autosite; craniopagus parasiticus is very rare “(additional head)”. A little more frequent is parasitic thoracopagus or xiphopagus. Pygomelus (a limb or finger or some other tissue emerging from the sacral area) is the most frequent type of these malformations.
Parasite can be either external or internal, as a skin-covered cyst (inclusio fetalis).